1. Field of the Invention
The present invention pertains to remote sensing, and, more particularly, to passive range finding with measurements in a single time sample.
2. Background of the Invention
A significant need in many contexts is to determine the relative position between two objects. For instance, in a military context, it may be desirable to determine the relative distance along a line-of-sight or range from a vehicle to a target. Several methods have been used to determine the range to a target with electromagnetic waves. The most common is to actively transmit a pulse of electromagnetic energy that travels at the speed of light and record the time at which the reflected pulse returns. Since the speed of this energy is the constant speed of light, this constant velocity multiplied by half the measured time difference would give the range between the target and the vehicle generating the energy pulse.
This is the basis for RADAR, which is an acronym for Radio Detection And Ranging. Laser range finders operate on the same fundamental principle but at a higher frequency of electromagnetic radiation. This approach is known as an active approach since it requires the active participation of the vehicle in generating the electromagnetic pulse so that the time and location of the transmitted energy source is known. This active approach may not be desirable in some contexts due to the cost of the radiating source. In other contexts, the active approach may not be desirable due to the expulsion of radiating energy betrays efforts to be stealthy.
A traditional method to passively measure range involves receiving a signal from a target radiating energy over time. Multiple measurements are required over time while the relative geometry between the target and the passive-sensing vehicle are changing. Accounting for the time differences in the trajectory path, the range of the target can be calculated through triangulation. The radiated energy returned from the target can be generated by the target or the target can be illuminated by a remote source where the location and time of energy generation is unknown.
A distinguishing feature of this approach is that multiple measurements are required over a period of time with changing geometry. If the sensing vehicle travels directly toward the target or at a constant bearing relative to the target, the relative range is unobservable using the traditional passive approach. This passive approach may not be desirable in some contexts due to the time required to travel a trajectory necessary for triangulation. In other contexts, the passive approach may not be desirable due to the inefficiency of the trajectory required for triangulation or the non-constant bearing trajectory.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.